1. Field
Embodiments described herein relate generally to a nonvolatile semiconductor memory device.
2. Description of the Related Art
There is a rapidly increasing demand for a NAND flash memory with an increase in a use of large-capacity data such as images or moving pictures in mobile apparatuses or the like. In particular, the use of a multi-value storage technique capable of storing information of two bits or more in one memory cell enables a large amount of information to be stored in a small chip area.
In highly integrated flash memories where memory cells are further miniaturized, when reading memory cells in which a data write operation is finished, a read pass voltage applied to non-selected memory cells affects a threshold voltage representing the data of a non-selected memory cell. This influence appears as change in a threshold voltage distribution. In particular, when a multi-value storage scheme is employed, the width of the threshold voltage distribution and interval between the threshold voltage distributions are set to be narrower than those in a 2-value storage scheme. Thus, the change in the threshold voltage distribution has a significant influence on the reliability of data. Therefore, it is necessary to execute a read operation capable of suppressing the change in the threshold voltage of a non-selected memory cell caused by a read pass voltage.